Red Tide FAQ

Red tides can last as little as a few weeks or longer than a year. They can even subside and then reoccur. The duration of a bloom in nearshore Florida waters depends on physical and biological conditions that influence its growth and persistence, including sunlight, nutrients and salinity, as well as the speed and direction of wind and water currents.

When do they most often occur?

Red tides most often form in late summer and early fall and persist through winter to spring; see our table Over 100 years of Red Tides off Florida’s West Coast.

Are red tides lasting longer?

Some considered the November 2017 to February 2019 red tide bloom in Florida to be the longest they could remember. While this bloom was persistent, we have had longer blooms on record.

Yes, many algae species cause red tides all over the world. Yet, the organism that causes Florida's red tide, Karenia brevis, is found almost exclusively in the Gulf and occurs from Mexico to Florida. K. brevis can be transported around the Gulf as coastal waters move with winds and currents. Some red tides have even been carried by the Gulf Stream current into the Atlantic Ocean as far north as Delaware.

Red tide in Florida can be found in bays and estuaries but not in freshwater systems such as lakes and rivers. Because Karenia brevis cannot tolerate low-salinity waters for very long, blooms usually remain in salty coastal waters and do not penetrate upper reaches of estuaries. However, other harmful algae, including cyanobacteria (blue-green algae), typically bloom in freshwater lakes and rivers.

Although the occurrence of a red tide cannot be predicted, scientists can forecast its movement using wind and water current data once a bloom is located. Scientists also monitor the concentration of the red tide organism by collecting water samples routinely and in response to blooms. Red tide movement and concentration are important because the effects of a red tide, such as dead fish and human respiratory irritation, depend on these factors. The information provided by forecasting and monitoring allows people to make informed decisions regarding their beach-going activities.

Many red tides produce toxic chemicals that can affect both marine organisms and humans. The red tide organism in Florida, Karenia brevis, produces brevetoxins that can affect the central nervous system of fish and other vertebrates, causing these animals to die. Wave action can break open K. brevis cells and release these toxins into the air, leading to respiratory irritation. For people with severe or chronic respiratory conditions, such as emphysema or asthma, red tide can cause serious illness. The red tide toxins can also accumulate in molluscan filter-feeders such as oysters and clams, which can lead to Neurotoxic Shellfish Poisoning in people who consume contaminated shellfish.

Red tides in Florida develop 10-40 miles offshore, away from man-made nutrient sources. In contrast to the many red tide species that are fueled by nutrient pollution associated with urban or agricultural runoff, there is no direct link between nutrient pollution and the frequency or initiation of red tides caused by Karenia brevis. However, once red tides are transported inshore, they are capable of using oceanic and land-based (including man-made) nutrients for their growth. Red tides occurred in Florida long before human settlement, and severe red tides were observed in the mid-1900s before the state’s coastlines were heavily developed. No single factor causes blooms of K. brevis. Blooms form as a result of the interactions between biology, chemistry, and ocean currents that unite nutrients with light and carry red tide to the beach. 

Control of red tide in Florida is not a simple issue. The harmful effects of a red tide are caused by toxins released by the organism. Potential controls must not only kill the red tide organism but also eliminate the toxins from the water. To date, this has not been possible; however, researchers are identifying ways to reduce shellfish toxicity. In addition, any control strategy must not harm the environment. In the 1950s, U.S. Fish and Wildlife Service and State of Florida scientists used copper sulfate to attempt to eliminate a red tide in coastal Florida waters. Although the copper sulfate killed some of the red tide cells, it led to the release of toxins that, along with the copper sulfate, had negative effects on other marine organisms. Controls must also be practical. Red tides vary greatly in size – expanding as far as 10,000 square miles – and can be present from the surface of the water to the seafloor. Presently, there is no practical and acceptable way to control or kill red tide blooms.

Some people experience respiratory irritation (coughing, sneezing, and irritation of the eyes, nose, and throat) when the red tide organism, Karenia brevis, is present and winds blow onshore. Winds blowing offshore may keep respiratory effects experienced by those on the shore to a minimum. Sensitive individuals (e.g., children, the elderly, those who are immunocompromised, and those who have chronic respiratory conditions) may be at risk of experiencing more severe symptoms. See Red Tide: Health Impacts on the Florida Department of Health’s Harmful Algal Blooms page for specific health-related recommendations. Also, check FWC’s Red Tide Daily Sample Map for the most recent information on blooms in your area.

Red tide can cause skin irritation, rashes, and burning/sore eyes. Do not swim in or around red tide. See Red Tide: Health Impacts on the Florida Department of Health’s Harmful Algal Blooms page for specific health-related recommendations.

Store-bought and restaurant-served shellfish are generally safe to eat during a bloom because the shellfish are monitored by the government for safety. Commercially available shellfish, if harvested locally, are tested for red tide toxins before they are sold. Watch the video Red Tide and Seafood Safety.

Your safest choice is to not harvest or eat molluscan shellfish or marine gastropods from red tide affected areas. In fact, recreational harvesting of bivalve mollusks such as hard clams, oysters and mussels from conditionally approved or approved shellfish harvesting areas is banned during red tide closures; these organisms may not legally be harvested during any closure of a shellfish harvesting area and should not be eaten. To determine whether or not harvesting of shellfish is permitted in an area, visit the Florida Department of Agriculture and Consumer Services, Division of Aquaculture website.

In addition, illegally harvested and unregulated shellfish are particularly dangerous and should never be consumed. For example, coquina clams and molluscan predators, such as whelks that feed on toxic bivalves, readily accumulate toxins in their tissues. An illegal harvest is a dangerous harvest.

Other seafood can be safely harvested during an event if the animals are not affected by the red tide organism and if properly prepared. For crabs, shrimp, and lobsters, do not eat the tomalley (green stuff, hepatopancreas). For scallops harvested during scallop season, eat only the muscle (white part). See Seafood Safety on the Florida Department of Health’s Aquatic Toxins page for more information.

Yes, if caught alive and healthy, finfish are safe to eat as long as they are filleted, and the guts are discarded. Rinse fillets with tap or bottled water. However, it is never a good idea to eat dead or distressed finfish (or any animals) from or near a red tide because the reason for the animal's strange behavior or death cannot be absolutely known. Watch the video Red Tide and Seafood Safety.

No. Cooking, heating, or freezing does not get rid of the toxins. Furthermore, the toxins do not affect the texture, taste, or smell. Watch the video Red Tide and Seafood Safety.

Just like people, animals may be affected by the red tide. If you live close to the beach, consider bringing outdoor pets inside during a bloom to prevent respiratory irritation. If you are at the beach with your pets, do not allow them to play with dead fish or foam that may accumulate on the beach during or after a red tide. If your pet eats dead fish, they may get sick. If your pet swims in the red tide, wash them as soon as possible. Most dogs lick themselves after swimming and will consume any toxins on their fur. Watch the video Red Tide and Pet Health and see the Department of Health Factsheet for more information.

Contact or visit a doctor or call the Florida Poison Information Center at 800-222-1222 for suspected exposure to harmful algae.

Florida Department of Health

Centers for Disease Control and Prevention

Florida Department of Agriculture and Consumer Services